Let's imagine that every food source except one suddenly becomes extremely toxic for humans and no matter what we try to do with them, they stay toxic.
I'd like to know if there's some food source humanity could survive on only by eating it, obviously no one is gonna be specially healthy by just eating from a single food source, but as long as it would allow humanity to survive for a long time, even if barely, it would be valid.
I think that potatoes are good as a single source that would allow for a typical person to survive even some months, but in a story that takes a lot of years (30-40 years) I don't think it would be enough, although if it were the case I'd like to know.
Is there any food source that would fulfill the mentioned condition? If so, which one?
Edit: As I see one answer that didn't fulfill my intention I'm explaining better. The food source must stay with whatever their natural nutrients were, any try of getting outside of that nutrients would end in toxicity.
There's also some magic involved in the story, so even if it's not realistic it can be considered this way.